1. Field of the Invention
This invention relates to an apparatus for determining an image position, wherein the position of a desired object image on a recording medium is determined on the basis of an image signal, which has been detected from approximately the whole surface of the recording medium and which represents the whole image recorded on approximately the whole surface of the recording medium. The desired object image having been recorded on a portion of the recording medium. This invention also relates to a method for adjusting read-out conditions and/or image processing conditions for a radiation image, wherein read-out conditions for a final readout and/or image processing conditions are adjusted on the basis of an image signal representing a radiation image in which a desired object image is embedded.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out. The X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-104645, and 55(1980)-116340, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, or on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-67240, 58(1983)-67241, and 58(1983)-67242. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order to approximately ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The term "image processing conditions" as used herein means a group of various factors, which are adjustable and set when an image signal is subjected to processing, which affect the gradation, sensitivity, or the like, of a visible image reproduced from the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
In the course of recording a radiation image of an object on a recording medium, it is often desirable for portions of the object not related to a diagnosis, or the like, to be prevented from being exposed to radiation. Also, when the object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality is adversely affected by the scattered radiation. Therefore, when a radiation image of an object is recorded on the recording medium, an irradiation field stop is often used to limit the irradiation field to an area smaller than the overall recording region of the recording medium so that radiation is irradiated only to that portion of the object, which is to be viewed, and part of the recording medium (i.e. the region inside of the irradiation field). The region inside of the irradiation field is often composed of an object image region, in which the image of the object is recorded, and a background region, upon which the radiation impinged directly without passing through the object. Of these regions on the recording medium, the region which it is necessary to reproduce is only the object image region. Therefore, when a visible image is to be reproduced from an image signal representing a radiation image, the shape and location of the object image region in the radiation image, which has been recorded over the whole area of the recording medium, should be determined, and image signal components corresponding to the object image region should be determined from the image signal. Appropriate read-out conditions for the final readout and/or appropriate image processing conditions should then be adjusted on the basis of the image signal components, which have thus been determined. In this manner, a visible reproduced image can be obtained which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. When the shape and location of the object image region are determined, characteristics of the radiation image depending on the structure of an image recording apparatus used to record the radiation image of the object, or the like, and the shape of the object are often taken into consideration. By way of example, a method for determining the position of an object image region in an X-ray image of the mamma of a human body is disclosed in Japanese Unexamined Patent Publication No. 61(1986)-170178. With the disclosed method, a change in the value of an image signal at the boundary between an object image region (i.e. a mamma pattern) and a background region is detected by utilizing such characteristics that the mamma pattern is recorded in an approximately semicircular shape on a recording medium and the background region is located outward from the circular arc, which defines the boundary of the approximately semicircular mamma pattern.
However, the methods described above are based on the assumption that the relationship between the position of the contour of an irradiation field and the position of an object image region in the region inside of the irradiation field, or the like, coincides with a predetermined condition. Specifically, based on such assumption, operations are carried out on an image signal in order to find whether each of a plurality of partial regions on the recording medium, which are expected as falling within the region inside of the irradiation field, falls or does not fall within the region inside of the irradiation field. Therefore, it often occurs that the shape and location of the object image region cannot be determined by taking the shape of the object, or the like, into consideration. Such problems occur when the relationship between the position of the irradiation stop and the position of the object differs slightly from the predetermined relationship or when patterns of characters formed of lead having a low radiation transmittance are recorded together with the object image on a recording medium. If the shape and location of the object image region cannot be determined, the read-out conditions for the final readout and/or the image processing conditions, which are appropriate for the object image region, cannot be determined. In such cases, for example, the read-out conditions for the final readout and/or the image processing conditions are determined on the basis of the image signal detected from the entire area of the recording medium, including the region outside of the irradiation field, which region was exposed to little radiation. Specifically, the read-out conditions for the final readout and/or the image processing conditions are determined such that the whole radiation image may have comparatively good image quality in a reproduced visible image. As a result, a reproduced visible image is obtained wherein the image density of the object image region is markedly high (or the luminance of the object image region is markedly low when the reproduced image is displayed on a CRT display device, or the like). Such a reproduced image is not suitable for the viewing purposes.